The present invention relates to a hammer drill or percussion hammer.
Such hammer drills have been known in the art. One of such hammer drills, is an impact wrench which has been disclosed in U.S. Pat. No. 3,874,460. In this known impact tool an impact energy of the striker is transmitted to the tool; the striker is reciprocally displaced in the inner space of the cylinder by the hollow piston, which serves as an actuating member, by means of the drive motor, so that during the operation an air cushion formed between the bottom of the hollow piston and the striker acts on the striker and when the tool is inoperative an underpressure air cushion formed between the bottom of the hollow piston and the striker acts on the striker in the opposite direction.
In the impact tool disclosed in GB-PS No. 16 00 944, impact energy is transmitted to the tool by a striker which is slidingly guided in the cylindrical inner space of the guide tube; the striker is axially reciprocally displaced by the drive motor via the piston which serves as an actuating member. During the operation the striker is taken along by a closed air cushion and in the inoperative position an underpressure cushion, which is formed between the piston and the striker, urges the striker in the opposite direction.
British specification GB-PS No. 14 67 215 discloses a striking mechanism with a guide cylinder in which a hollow cylinder having a transverse wall and a piston therein is guided so that the hollow cylinder forms a striker, the piston of this hollow cylinder being reciprocally driven from a motor via a crank transmission. A periodically open or closed to the atmosphere air chamber is formed between the transverse wall of the hollow cylinder and the front wall of the piston; the air filling that air chamber acts as a spring and transmits a drive load to the striker and, after the impact, during the reverse stroke of the reciprocal piston the striker is pulled back under the underpressure.
In these known striking mechanisms only one air chamber is provided between the reciprocating actuating member and the striker. In this case a relatively high pressure or underpressure should be produced in the air chamber in order to provide a normal operation of the striking mechanism in the direction of the transmitting impact energy to the tool and in the reverse direction as well. This, however, causes strong vibrations of the whole power tool during the operation, which leads to very undesired oscillating loads on the operator.
Applicant's U.S. Pat. Nos. 4,310,055 and 4,336,848 also disclose hand-held impact tools in which one air cushion is utilized in the striking mechanism.